Carrier-wave-transmission system



J. s. JAMMER Filed Maren 4, 1924 CARRIER WAVE TRANSMISSION SYSTEM L1' LaFe'b. 15 1927.

IIIII tems.

y Patented eb. A15,1927.

UNITED ,STATES JACOB SUTER JA'MMEB, 0F NEW YORK, N. Y., ASSIGNOR T0WESTERN ELECTRIC COM- PANY, INCORPORATED, OIEl NEW YORK, N. Y., Al(.'IORPORATION OF NEW YORK.

CARRIER-WAYE-TRANSMISSION SYSTEM.

Appncaaon fiiea March 4, 1924. serial No. 696,775.

The present invention relates to the regulation ,oftransmission in anelectrical system of distribution, and has part1cular applicatlon tocarrier Wave transmission sys- It is an object of the invention toenable regulation of the transmission through such a system in aneflicient and convenient manner. v

'A feature of the invention comprises transmission controlling devicesarranged to give the desired transmission level for the entire channelvor line and to permit changes in the transmission level to be readilymade sate for line changes.

This will be more clearly understood if it be assumed that a certainover-all transmissionlevel is to be maintained between the two distantlyseparated low frequency terminals of a carrier wave channel. Such achannel, as is well known, includes a high frequency transmissionsection and portions over which low frequency currents, such as speechor other signal currents are transmitted. Variations in the transmission.characteristics of the channel (due to weather or other conditions) arecompensated for by potentiome- `ters vor other adjustable transmissioncontrol 30 devices. These control devices are commonly situated at suchapoint kin the system that they vary the high frequency load on thedetecting and receiving circuit. Over a limited range of load values`changes in the high frequency load produce correspondingl changes in thedetected output current. For other loads, however, considerablyl outsidesuch range. an altogether different relation exists between the changesin high frequency load and the detected output. For example, for acertain region of load` a change in the high frequency loadcorresponding to a change of one mile of standard cable in thetransmission equivalent may produce a change in the detected output alsocorresponding to a one mile standard cable change in lou7 frequencyequivalent. With a differient value of load, however. it has been foundin practice that a one mile change in high w frequency load produced asmuch as a thirtymile change in thesdetected output,lwhile at stillothe'values of high frequency load. a thirty mile Vchange in highfrequency load was required to produce aone-mile change in detectedoutput.

' for regulation purposes such as to compen- It is seen, therefore, tobe advantageous to "fix the hlgh frequency load at such' value that thepotentiometer or other input control all transmission equivalent of asystem to some specific value. Forinstance. if a channel has a zeroequivalent, the low frequency detected arrival current has the sameamplitude as the current which is put into the channel at the distantterminal. This means, of course, that the gain introduced by the variouscircuit elements ofthe channel just make up for the transmission losses.Since the transmission characteristics of lines differ widely amongthemselves and the characteristics of the same line may differ Widelywith conditions` and also the channel transmission characteristicsdiffer widely'among channels on the same line, it will often, if notvgenerally, happen that the transmission level of a channel cannot bebrought to the required value by changing,

the high frequency load on the receiver withi out carrying the loadoutside the relatively small region discussed above in which the desiredrelation exists between load change and change in detected output. 4Thisdiiiculty is overcome in accordance with the invention by providing alow frequency transmission control element which is adjustable to changethe strength of the received detected current within relatively widelimits. This element, being in the low frequency circuit, does notdisturb the high frequency portion of the circuit, so that with thearrangement provided by the invention, the desired fine adjustment canbe effected by the high frequency input control means.

These and the other features and objects of the invention will be moreclearly understood from the following detailed descripdrawing.

Fig. 1 is a schematic circuit drawing of so much of a carrier wavesystem as is necessary to an understanding of the invention.

Fig. 2 shows curves illustrative of the action of the regulating devicesof the invention.

In Fig. 1 there is shown the west terminal of a multiplex carriertransmission system. This System is of the general type shown in Fig. 42of the paper by Colpitts and Blackwell on Carrier current telephony andtelegraphypublished in vol. 40 of the Journal of the American Instituteof Electrical Engineers in a series of installments beginning in April,1921, and reference may be had to that figure for a more detaileddisclosure of the actual circuit arrangements of the various parts ofthe system. In Fig. 1 of the drawing, a carrier line ML is shownterminating in two grouping filters GF1 and GF2 which serve to separatethe oppositely directed carrier waves. For example, all of the carrierwaves employedfor transmission from the station shown in Fig. 1 to themain line ML may be comprised in an upper frequency range and these areselectively transmitted by the filter GFI. The carrier waves received atthe station in Fig. 1 from .the main line ML maybe comprised in a lowerrange to which the filter GF2 is made selective. If preferred, theseparation between transmitted and received waves may be effected by theused of a hybrid coil as shown in Fig. 42 of the Colpitts and Blackwellpaper.

In Fig. 1, three transmitting channels and three receiving channels areindicated schematically. The transmitting filters comprise a modulator Mandl a band filter MBF for transmitting from the ymodulator to the linethe desired components of the modulated wave such, for example, as thecarrier and one of the side bands. Each receiving channel, likewisecomprises a band filter DBF for selectively transmitting the componentsintended for the particular channel and a demodulating circuit D forderiving from the received waves the low frequency signal componentssuch as speech. In the drawing, the apparatus belonging to each channelis designated by a suitable subscript.

The terminations of the low frequency lines adapted for connection withthe carrier system are not shown in detail in Fig. 1 but are merelyindicated by the jacks 1 to 6 inclusive and the respective linebranches, such as L1 to L3 indicating the transmitting line branchesandLl to L3 indicating the receiving line branches. The association ofthese low frequency line branches with the low frequency telephone linesmay be made in any suitable manner, such, for example,l

as that indicated in Fig. 42 of the Colpitts and Blackwell paper.

One of the receiving channels is illus- -pad 10 may be omitted in somecases.

trated more in detail in Fig. 1 and this will now be described inconnection with the devices for regulating the transmission in thechannel in accordance with the invention. The detectingcircuit\generally indicated at D8 comprises the high-frequency pad 10, apotentiometer 11, the amplifier 12, the detector 13 and selectingcircuits 14 and 15 for separating the detected speech from the in the U.S. Patent to Venues, No. 1,472,501,

issued October 30, 1923. i

The operation of this portion of the circuit will be clear from thedrawing and from the above designation of the various l portions of thecircuit. The high frequency Waves selectively transmitted to the filterDBF 3 are impressed on the amplifier 12 and the detector 13 with adesired amplitude as determined by the settings of the hgh frequency-pad10 and the potentiometer 11. As explained hereinafter the high frequencyThe detector 13 serves in well known manner to producein its outputcircuit low frequency variations corresponding to the signal beingtransmitted over the channel, such, for example, as speech variations.selectively transmitted through the low pass filter 15 which is sodesigned as to prevent passage of currents of higher frequencies such asthe carrier frequency. The speech currents after traversing the filter15 pass through the low frequency pad 1G to the receiving jack 17 fromwhich they may be impressed on the desired low frequency line by theinsertion of the proper plug into the jack 17.

The currents of the carrier frequency appearing in the output of thedetector 13 are selectively transmitted through the high pass filter 14to the rectifier 18. This filter 14 is designed not to pass currents ofspeech frequencies. Only current of the carfier frequency is thereforeimpressed on the detector 18 and rectified high frequency currentappearing in the output circuit is indirlhese are cated by a suitablemeter 19 and is utilized to actuate the relay 20." The relay 20 may beused in accordancewith known practice to transmit a call signal to theoperator at the receiving terminal at the station shown in Fig. 1 or maybe relayed to a further point. For example,` if the operator at thedistant station interrupts the normally transmitted carrier wave, relay20 becomes deenergized and closes a circuit through the calling lamp 2lto indicate to the operator ofthe station in Fig. l that a call isincoming on channel 3.

Itv will be understood that each of the other' receiving channels may bearranged in substantial duplicate of thei channel 3 so that eachreceiving channel will haveits individual high frequency and lowfrequency pads corresponding to 10 and 1G. its individual amplifying,detecting and rectifying devices. calling relay and calling lamp. Eachof the transmitting channels is adjusted and 'supervised either by anattendant or in any known manner so that the output current ismaintained at a constant value. The only variations in the receivedcurrent strength are therefore those due to variations in transmissioncharacteristics of .the system.

As indicated above. the strength of the detected speech currents willvary in some non-linear relation with the high frequency load impressedon the detecting circuit. Re

ferring to Fig. 2. for example, the full linecurve A may indicate therelation between the high frequency arrival current at the terminals ofthe band filter DBF3 and the strength of the detected speech currents atthe terminals of jack 17 as compared with the strength of the speechcurrents impressed on the transmitting jack at the distant ter min-al ofthis station. That is. values of low frequency equivalent above the zeroaxis indicate a gain in overall 'transmission between the transmittingjack at the distant station and the receiving jack 17 atl the station ofFig. l. while values of .low frequency equivalent belowthe Zero axisindicate an over-all loss. As the high frequencv load impressed on thedetector'is varied in any manner due to the high frequency line pad l0or the potentiometer 11 or the transmission line ML, the low frequencyequivalent of the Asystem is changed by the relation shown by the curveA.

The main function of the potentiometer 11 is to secure a fine adjustmentof the low frequency output detecting circuit to compensate for thevariations in the transmission characteristics of the line ML whichoccur from time to time and which are in general relatively small. It isconvenient in practice to set the potentiometer l1 at 'about itsmidpoint for an average condition of the line ML so that thepotentiometer will give a substantially equal variation in eitherdirectionfrom the average condition to. compensate for correspondingvariations in the transmission `characteristics of the main line. If itbe assumed that a 4zero transmission: equivalent is desired between thelow frequency yterminals of the channel a'the high frequencyloadimpressed on the receiving terminal when the main line ML has itsnormal characteristics should be that indicated by the dotted line 25 inFig. 2. If the system is adjusted to have a zero low frequencyequivalent for this normal characteristic of the main line and withthe'potentiometer adjusted to its midpoint, the conditions will be asindicated in Fig. 2 with the potentiometer in position I. The range ofvariation of the high frequency load by the potentiometer is indicatedin this figure by the length of the horizontal line markedpotentiometer. The amount of variation in the low frequency outputproduced by varying the potentiometer setting from its midpoint ineither direction to the left or to the right is shown by thecorresponding ordinates of the curve A. Within the range of thepotentiometer, therefore, the low frequency equivalent may be eitherincreased or decreased in a substantially linear manner as shown by thecurve A Within these limits.

In setting up a system or in adjusting a system to proper workingconditions it may be found that varying the potentiometer 11 from oneend of its scale to the other produces practically no effect in thedetected speech currents. It may also happen that the detected 'speechcurrents are much stronger than is desired, that is, the overall gainbetween the transmitting low frequency jack and the receiving lowfrequency jack is too great.

This condition may be as represented in Fig. 2 by position II of thepotentiometer ll and by the curve A. On account of the small slope whichthe curve A has in its upper right hand portion, no amount of adjustingthe potentiometer is effective in producing any substantial change inthe detected. output. This is due to thefact that the high frequencyload iinpressed on the detecting circuit is too great. By inserting ahigh frequency artificial line or pad l() such as to introduce a loss inthe high frequency channel, the effect of the potentiometer incontrolling the low frequency output may be changed from that indicatedin position II to that indicated in position III. In position III, thepotentiometer gives a substantially linear regulation in the detectedlow frequency output. If still greater losses are introduced in the highfrequency arrival currentsq the effect of the potentiometer 11 may bestill further increased since' its position with respect` to the curve Awill be moved further to the left. However, no amount of change in thehigh frequency pad l0 will bring about the relations indicated by curveA andby the potentiometer in position I, and it will he impossible.therefore, by the use of the high frequency pad alone to adjust thesystem to `give a zero low frequency equivalent and atthe same time givea fairly linear relation between the potentiometer setting and theoutput current withI the desired sensitivity of the potentiometersettings.

In accordance with the invention, thedesired regulation is effected byintroducing enough losses by the highvfrequency pad to reduce the meanhigh frequency load on the amplifier to the value indicated by line ofFig. 2. The required low frequency equivalent is then secured byintroducing a low frequency pad 16 which produces the required lossbetween the detector output and the jack 17 to reduce the strength ofthe detected speech currents to the value indicated by the zero axis inFig. 2. By means of the high frequency pad 10 and the low frequency pad16, therefore, the regulation may be readily changed from that indicatedby'the potentiometer at position II and curve A to that indicated byposition I and curve A.

Y The low frequency pad 16 does not, of course, affect the highfrequency load on the detecting circuits. The use of the separate highfrequency pad and low frequency pad enables the high frequency loadimpressed on the detecting circuit to be adjusted to the point at whichthe detector operates most efficiently without regard to the actualstrength of the detected currents. The latter may be regulated by meansof the low frequency pad 16.

The curve B in Fig. 2 indicates the relation between the high frequencyarrival current and the readings of the meter 19, that is, the rectifiedcarrier component. With the high frequency load indicated by the curve Ait is seen that curve B is'practically symmetrical with respect to theline 25. For a considerable region each side of the line 25 both curve Aand curve B have slopes of about This relation permits the attendant tojudgev of the transmission characteristics of the main line ML from timeto time by observing the reading of the meter 19. If the regulation Werethat corresponding to the curve A with the potentiometer at position,II, ythe attendant would not observe a substantial change in the readingof the meter 19v by any change in setting of the potentiometer 11. Thevariation in the meter reading With a change in setting of thepotentiometer 11 may, therefore, be taken as a convenient indication ofthe proper regulation of the channel.

The use of the high frequency and loW frequency pads, as described, notonly per- ,mits the desired transmission level to be seunder thel thehighest frequency channels. The separate high frequency and lowfrequency regulating pads readily permit all channels regardless oflength or frequency employed to be broughtI to the same over-alltransmission level and still keep the potentiometer settings the same. Agiven change in the potentiometers, Whether made manually orautomatically, and separately or in common,

will produce substantially the same change in detected output and Willtherefore serve to compensate in all channels for a given change in lineattenuation.

lVhat is claimed is:

1. In a system in which high frequency Waves are transmitted betweendistantly separatedv stations V,and are detected to yield desired lowfrequency components, the method of regulating transmission comprisinglowering the high frequency amplitude, rcgulatingthe high frequencyamplitude bcfore detection to obtain a required relation between changesin the high frequency amplitude and the detected low frequency current,and independently regulating the -detected low frequency current toobtain the required low frequency amplitude.

2. In a carrier Wave transmission system, a line the transmission levelof which is subject to change, a receiving carrier Wave channel,including a detector and a low frequency termination, means associatedwith said detector for regulating the high frequency load impressed onthe detector to compensate for changes in the line transmission level,means to control the sensitivity of said regulating means, and means inthe low frequency terminating circuit for independently regulating thedetected output current.

3. In a regulating system for a carrier signaling channel having adetecting circuit, a low frequency output circuit, a high frequencyinput circuit and an adjustable potentiometer for varying the highfrequency load impressed on the detecting circuit` the combination `ofan artificial line in the high frequency input circuit for controllingthe sensitivity of the potentiometer, and an artificial line in the lowfrequency output circuit for determining the over-all transmission levelof the channel.

4. In a multiplex carrier Wave transmission system, a line. a pluralityof carrier channels supcrposed on the line, a detector ineach receivingchannel for deriving low frequency signal currents from the transmittedhigh frequency Waves, an adjustable potentiometer individual to eachchannel for controlling the high frequency load on the respectivedetector and the resultant low frequency output of the detector, anartificial line individual to each channel between the multiplextransmission line and the respective potentiometer for making thesentector for adjusting the low frequency output to the same level inall of the channels for a substantial mid-scale potentiometer setting.

5. In a regulating system for a carrier 10 signaling channel having adetector and a vhigh frequency input and 10W frequency output therefor,the combination of means in the high frequency input circuit forlowering the high frequen'c for regulating the high frequency amplitudein the input circuit, and means for independently regulating the lowfrequency amplitude in the low frequency circuit.

In Witness whereof, I hereunto subscribe amplitude, means my name this29th day of February A. D., 20

J ACOB SUTER J AMMER.

